The use of polyalpha-olefins or copolymers thereof to reduce the drag of a hydrocarbon flowing through a conduit, and hence the energy requirements for such fluid hydrocarbon transportation, is well known. These drag reducing agents, or DRAs, have taken various forms in the past, including slurries of ground polymers to form free-flowing and pumpable mixtures in liquid media.
In general, the DRA polymer may be obtained via solution polymerization of an alpha olefin monomer, or a mixture of olefinic monomers, or from bulk polymerization (i.e., without solvent) of such monomer(s). The DRA polymer may then be subsequently made into particulate form by cutting, chopping, granulating, and/or grinding, at cryogenic or ambient temperatures. Alternatively, it may be precipitated from solution by addition of a non-solvent component. Mixtures of polymer solids from both sources may be used.
A problem that has been frequently addressed in the art, however, is that there is a natural tendency for such slurries, containing the particulate polymer DRA, to settle over time, or to separate or agglomerate such that the slurries no longer maintain a free-flowing and pumpable nature. A generalized description of how this problem has been addressed is that the slurry of polymer DRA and liquid carrier also frequently includes a partitioning agent, a wetting agent, and/or a rheology modifier. These three components, which are frequently all included, are referred to generally as “suspension aids”. The purpose of the partitioning agent is to physically hold the polymer DRA particle surfaces apart. The purpose of the wetting agent is to wet the polymer DRA surface, and the purpose of the rheology modifier is to increase the viscosity of the liquid carrier to slow down polymer DRA particle settling or rising. In some cases a single ingredient may serve multiple purposes within the suspension aid package.
The liquid carrier is, in some embodiments, a non-solvent for the polymer DRA and may vary widely. Selections for this component may include both aqueous and non-aqueous liquids, including, for example, water and aqueous solutions of varying pH and ionic strength; alcohols and fatty alcohols; glycols and diols; glycol ethers; glycol esters; mixtures of these; and the like.
The wetting agent is virtually always included in such formulations. Without a wetting agent the liquid carrier would quickly drain away from a non-wetted polymer surface. This would result in a highly separated slurry. Fatty acid waxes have been used as wetting agents, as well as commercially available surfactants, such as Span™, Tween™, Brij™, and Myrj™. These surfactants, which are generally sorbitan esters, ethoxylated sorbitan esters, alcohol ethoxylates and polyoxyethylene fatty acids, are available from Uniqema.
Inclusion of a partitioning agent is also generally desirable in these slurries. This is because polymer DRNs are often in the form of soft, tacky particles that will agglomerate, or “cold flow”, when their unaltered surfaces come into contact with one another. Again, fatty acid waxes are often used as partitioning agents, as well as polyolefin homopolymers and copolymers of various densities; oxidized polyethylene: polystyrene and copolymers; carbon black and graphites; precipitated and fumed silicas; natural and synthetic clays, and organo-clays; aluminum oxides; talc; boric acid; polyanhydride polymers; sterically hindered alkyl phenol oxidants; magnesium, calcium and barium phosphates, sulfates, carbonates and oxides; mixtures thereof; and the like.
A rheology modifying agent may, in some embodiments, also be added to reduce separation of the polymer DRA slurry. By adding the modifying agent to the liquid carrier, settling or rising of the DRA polymer may be hindered or prevented. Some rheology modifiers, such as modified cellulosics and natural gums, may be heated to effect complete dissolution and may also function as wetting agents. However, these have no particulate properties that would render them capable of also serving as partitioning aids. A further disadvantage to this approach is that the higher viscosity automatically reduces the flowability properties of the polymer DRA slurry.
Another practice employed to stabilize polymer DRA slurries is to match the suspended particle density to the liquid carrier density, by using a combination of carrier components in appropriate proportions. The drawback to this practice is that there are relatively few available selections for carriers that are economical, have the desired densities, and also exhibit desirable hydrophobic or hydrophilic properties.
In view of the above, there is still a need in the art to discover ways to produce stabilized polymer DRA slurries that are convenient and economical and which do not suffer from the drawbacks listed hereinabove.